The present invention relates to an improved process for the polymerization of alpha-olefins as well as to catalyst systems comprising solid complexes which can be used for the polymerization of alpha-olefins, a process for preparing these solid complexes and the polymers obtained by means of the process of the invention.
It is known to use catalyst systems comprising a transition metal compound and an organo-metallic compound for the low pressure polymerization of olefins. Belgium Pat. No. 791,676 of Nov. 21, 1972, describes catalyst systems of which one constituent is obtained by reacting, with one another, (1) an oxygen-containing organic compound (M) of a metal, such as a magnesium phenate or a magnesium alcoholate, (2) an oxygen-containing organic compound (T) of a transition metal and (3) an aluminum halide (A). These catalyst systems possess the extremely important advantages of having very high activity and productivity while their preparation is extremely simple and does not lead to any contaminating by-product. Finally, and above all, the polymer obtained has very good morphological properties which makes it possible to carry out a continuous suspension polymerization with a very high polymer content and, hence, a very low relative amount of diluent which normally requires treatment before recycling.
Because of the above advantages, it would thus be highly desirable to be able to use the catalyst systems such as described in Belgium Pat. No. 791,676, for the manufacture of all types of polymers. However, these systems are difficult to use for the manufacture of polyolefins having a wide molecular weight distribution. In fact, to form such polyolefins, it is essential either to use catalyst systems wherein the atomic ratio of transition metal/metal of compound (M) is low or to carry out the polymerization with a relatively high concentration of hydrogen. The first method gives rise to considerable difficulties in the preparation of the catalyst system and therefore, loses the advantages mentioned above. The second method results in a rise in the melt index of the polymer and therefore, also cancels out the advantages desired. Further, this latter method results in an excessively large decrease in the productivity of the catalyst system.
A large proportion of polyolefins produced by low pressure polymerization are used in processes, such as extrusion and extrusion blowing, which require the polymer to have a wide molecular weight distribution. A catalyst system is, therefore, desired which is capable of readily producing polyolefins having wide molecular weight distribution and which has all of the essential advantages of the above mentioned system.